Rockbox as an application: add get_user_file_path().

For RaaA it evaluates user paths at runtime. For everything but codecs/plugins it will give the path under $HOME/.config/rockbox.org if write access is needed or if the file/folder in question exists there (otherwise it gives /usr/local/share/rockbox).
This allows for installing themes under $HOME as well as having config.cfg and other important files there while installing the application (and default themes) under /usr/local.

On the DAPs it's a no-op, returing /.rockbox directly.

Not converted to use get_user_file_path() are plugins themselves, because RaaA doesn't build plugins yet.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@27656 a1c6a512-1295-4272-9138-f99709370657

apps/gui/theme_settings.c

@@ -97,7 +97,9 @@ void settings_apply_skins(void)
         CHART2(">skin load ", skins[i].suffix);
         if (skins[i].setting[0] && skins[i].setting[0] != '-')
         {
-            snprintf(buf, sizeof buf, WPS_DIR "/%s.%s",
+            char path[MAX_PATH];
+            snprintf(buf, sizeof buf, "%s/%s.%s",
+                     get_user_file_path(WPS_DIR, false, path, sizeof(path)),
                      skins[i].setting, skins[i].suffix);
             skins[i].loadfunc(screen, buf, true);
         }